FM stereo decoders perform three primary functions. First, the decoder must detect and lock onto the 19kHz pilot signal that is added to the FM composite for synchronization of the transmitter and receiver. Second, the decoder multiplies the FM composite signal by a 38kHz signal that is in phase with the original 19kHz pilot. This multiplication process permits separation of "Left+Right" and the "Left-Right" information with simple low-pass filters. Third, L+R and L-R signals are added and subtracted to give the desired "Left" and "Right" information fed to the speakers of the FM stereo receiver.
It is desirable to eliminate the pilot energy from the signal path in the FM stereo receiver for two reasons. First, the 19kHz signal can be heard by some listeners, and second, the presence of the pilot may cause the audio amplifiers to clip during large signal conditions.
The standard approach to eliminating the pilot is to place a high Q notch filter in the signal path. This approach can cause problems in both the stop band as well as the pass band. Due to component mismatch and temperature effects, the accuracy of the center frequency of the notch is, in general, poorly controlled. This results in unpredictable pilot rejection. In the pass band, the notch filter can cause gain errors which result in the L+R amplitude varying from that of the L-R amplitude. This gain error results in a loss in stereo separation. Because of the circuit complexities, many decoder systems do not even attempt to cancel the pilot.